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Tectonic and magmatic evolution of the eastern Karakoram, India
Geodinamica Acta 12 (1999) 341–358 © 1999 Éditions scientifiques et médicales Elsevier SAS. All rights reserved S0985311199001011/FLA
Tectonic and magmatic evolution of the eastern Karakoram, India Rajeev Upadhyaya*, Anshu K. Sinhab, Rakesh Chandrac, Hakim Raid a b
d
Institute of Geology, ETH-Zentrum, 8092 Zurich, Switzerland Birbal Sahni Institute of Palaeobotany, 226007 Lucknow, India c Government Degree College, 176215 Dharamshala, India Wadia Institute of Himalayan Geology, 248001 Dehradun, India Received 2 September 1999; accepted 1 October 1999
Abstract – The Shyok suture zone separates the Ladakh terrane to the SW from the Karakoram terrane to the NE. Six tectonic units have been distinguished. From south to north these are: 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram terrane including the Karakoram batholith. Albian–Aptian Orbitolina-bearing limestones and turbidites of the Saltoro formation tectonically overlie high-Mg-tholeiites similar to the tectonically overlying Shyok volcanites. The high-Mg tholeiitic basalts and calcalkaline andesites of the Shyok volcanites show an active margin signature. The Saltoro molasse is an apron-like, moderately folded association of redgreen shales and sandstones that are interbedded with ~ 50 m porphyritic andesite. Desiccation cracks and rain-drop imprints indicate deposition in a subaerial fluvial environment. Rudist fragments from a polygenic conglomerate of the Saltoro molasse document a post-Middle Cretaceous age. The calcalkaline andesites of the Shyok volcanites are intruded by the Tirit granitoids, which are located immediately south of the Ophiolitic melange and belong to a weakly deformed trondhjemite-tonalite-granodiorite-granite suite. These granitoids are subalkaline, I-type and were emplaced in a volcanic arc setting. The subalkaline to calcalkaline granitoids of the Karakoram batholith are I-and S-type granitoid. The I-type granitoids represent a typical calcalkaline magmatism of a subduction zone environment whereas the S-type granitoids are crustderived, anatectic peraluminous granites. New data suggest that the volcano-plutonic and sedimentary successions of the Shyok suture zone exposed in northern Ladakh are equivalent to the successions exposed along the Northern suture in Kohistan. It is likely that the Kohistan and Ladakh blocks evolved as one single tectonic domain during the Cretaceous–Palaeogene. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture (50–60 Ma) together with tectonic activity along the Nanga ParbatHaramosh divided Kohistan and Ladakh into two arealy distinct magmatic arc terranes. The activity and a dextral offset along the
* Correspondence and reprints: [email protected]
Karakoram fault (Holocene–Recent) disrupted the original tectonic relationships. © 1999 Éditions scientifiques et médicales Elsevier SAS subduction / magmatism / collision / accretion / Karakoram
1. Introduction Closure of the Neo-Tethys ocean occurred along the Indus-Tsangpo suture zone between 60 and 50 Ma [1–3]. The Indus suture zone is marked by obducted remnants of the former Neo-Tethyan oceanic lithosphere along with Triassic to Eocene marine sediments [3, 4]. To the north of the Indus suture zone, in Ladakh, another important tectonic zone is documented by the rocks of the Shyok suture zone and the Karakoram batholith (figures 1 and 2). The Shyok suture is interpreted as an oceanic suture [5] or the relic of a back-arc basin [6]. Rai [7] argued against the existence of a subduction zone along the Shyok suture. In his interpretation, the Indus and Shyok zones are segments of one single suture which has been cut by the Ladakh batholith. Brookfield and Reynolds [8] and Reynolds et al. [9] suggested that the Shyok suture did not close before the Miocene and therefore considered it to be younger than the Indus suture. Srimal [10], however, suggested that the tectonic units accompanying the Shyok ophiolites were part of a collage between the northern margin of India and the southern margin of the Asian plate. Significant contributions have been made to the understanding of the tectonic evolution of the western continuation of the Shyok suture zone, the Northern suture exposed in northern Pakistan [11–15] (figure 1). It was postulated that the Northern suture resulted from collision between the Kohistan island arc and the Asian margin
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Figure 1. Geological map of the Shyok suture zone in Nubra-Shyok valley, Saltoro hills, northern Ladakh, India. K.K. Fault refers to the Karakoram fault; A–B = section of figure 2.
between 100 and 75 Ma [16]. A minimum age for the closure of the Northern suture is given by the northeast–
southwest trending Jutal-Nomal diorite dykes that yielded a Ar/40Ar age of 75 Ma [11] and cut across structures related
39
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Figure 2. Geological cross-section along line A–B on figure 2. KF refers to the Karakoram fault. Legend as on map (figure 1).
to the Suture. A maximum age is indicated by the youngest marine sediments, which are Aptian–Albian in age [17]. No detailed study, however, has been made to reconstruct the tectonic evolution of the Shyok suture in northern Ladakh [3]. Here we report new field observations on stratigraphy, sedimentology, and geochemical data of plutonic rocks of the Shyok suture zone. We outline its geology along a transect in the Nubra-Shyok river valleys. We describe a thick sedimentary-volcanic succession yielding Middle Cretaceous foraminifera. We also discuss the tectonic implications for regional correlation and comparison with the better investigated areas of northern Kohistan.
2. Geological framework of the Shyok suture zone in eastern Karakoram The rocks of the Shyok suture zone, trending northwest– southeast (figure 1) across the Nubra-Shyok valley, occur in intensely deformed tectonic slices between the Ladakh
batholith, to the southwest, and Karakoram batholith (figure 3) to the northeast. These tectonic slices comprise a variety of sedimentary, volcanic and plutonic rocks. Across a traverse through the Shyok-Nubra river valleys and the adjoining part of the Karakoram terrane, the major tectonic slices described in this paper are made up of the following rock units, from south to north i.e., from the structural bottom to top: 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram batholith. 2.1. Saltoro Formation The lowermost imbricate of the Shyok suture zone consists of the Saltoro formation (the Saltoro flysch [18, 19]) (figures 1 and 2). This formation begins at its base with a very low grade, gently folded calcareous succession which, to the south, is separated by a steeply dipping thrust from the Upper Cretaceous to Eocene Khardung formation of the Indus suture zone (figures 4 and 5) [20]. The lower part of
Figure 3. Panoramic view of the Nubra river valley showing the tectonic juxtaposition of the Shyok ophiolitic melange and the Karakoram batholith. View looking towards NE of the Charasa in figure 1.
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Figure 4. Field sketch cross-section across the Shyok suture zone near the village of Khalsar in figure 2. Tonalite refers to the Tirit granitoids.
the Saltoro formation is well exposed to the south of the village of Khalsar (figure 1) where it consists of thinly and mostly even-bedded, highly fissile and cleaved slates, phyllites, siltstones and intercalations of thinly to mediumbedded grey limestones and marbles. We have found a bryozoan fauna from a 5-m-thick, thinly bedded recrystallised limestone horizon (figures 1 and 6). These bryozoans were identified as Cheilostomata of possible Late Jurassic age (identification by Prof. A.K. Guha, I.I.T., Kharagpur). Other fossils are recrystallised echinoids, foraminifera and crinoid stems of undeterminable age. Near Horo Sosten (figure 1), we have identified a 150–200-m-thick limestone succession tectonically resting on volcanic rocks identified as high-Mg tholeiitic basalts of island arc affinity [21]. The partly recrystallised limestone contains abundant bivalves and other molluscs. Thin-section examination yielded rudists and a rich foraminiferal assemblage including Orbitolina discoidea, Palorbitolina and Texularia of Early to Middle Cretaceous age. This limestone horizon may be correlated with the Aptian–Albian Yasin Group in northern Kohistan [17] and with a volcano-sedimentary succession reported farther southeast near Pangong Tso lake (~150 km southeast of Khalsar, [22]). To the northeast, the fossilifer-
ous limestone is overlain with a tectonic contact by a ~1 500-m-thick volcanogenic sequence that is more calcareous near its base. The turbidites are interbedded with thinto medium-bedded black pyritic shales, grey to green fissile slate, siltstone and medium- to thick-bedded, medium- to coarse-grained compact sandstone to siltstone. The sedimentary structures include flute casts, graded bedding, parallel, wavy and convolute laminations (figure 7) and currentripples. 2.2. Shyok volcanite Under the heading of Saltoro volcanite we reunite different occurrences of volcanites of which we do not know the original palaeogeographic position. Sporadic outcrops occur below the Saltoro formation near Shukur (figure 1). Southeast of Diskit the Saltoro formation is tectonically overlain along a steeply dipping thrust by chlorite schists, basic volcanites and cherts of the Shyok volcanite (figures 1, 2 and 4). West of the Karakoram fault they also occur tectonically sandwiched between the underlying Saltoro molasse and the Ophiolitic melange, above. East of the Karakoram fault, they crop out between the villages of Panamik and Tirit and
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Figure 5. Detailed field sketch across the Nubra-Shyok valley showing the different tectonic imbricates of the Shyok suture zone near the Nubra and Shyok river confluence point in figure 2. Tonalite = Tirit granitoids; K.K. Fault = Karakoram fault.
near the confluence of the Nubra-Shyok rivers. These volcanites are the most widespread lithological unit in the Nubra-Shyok valley (figure 1). Rai [18] estimated that they are up to 4 km thick, consisting of a heterogeneous sequence of basalts and andesites with ignimbrites. The detailed geochemistry of the Shyok volcanites is beyond the scope of this paper, but preliminary geochemical data suggest that they may include different units. The volcanic rocks exposed near Shukur are fine- to mediumgrained, massive and highly fractured high-Mg tholeiites with rare vesicles; however between the villages of Kuri and Tirit and near the Nubra-Shyok confluence they are basaltic andesites. These andesites are medium-grained, strongly epidotised and agglomeratic in texture; vesicles are well developed, vary from a few mm to 1 cm in diameter and are filled with calcite, zeolites and chlorite. The basaltic andesites are also cut by hydrothermal veins rich in sulfide mineralisations, mainly chalcopyrite and flaky hematite. The preliminary geochemical data further suggest that, like the
high-Mg tholeiites the basaltic andesites are calcalkaline volcanites. No radiometric data are available. Since the Shyok volcanites are separated by faults from the surrounding rocks, we can only speculate about their age. The Lower–middle Cretaceous Orbitolina-bearing limestone from the Shukur village may provide a time constraint because we think that the tectonic contact near Shukur is not a major fault and, therefore, the volcanites may well be the base to the overlying limestones. Based on field relationships, regional distribution, similarity in composition and geochemical characteristics, it is suggested that the Shyok volcanites have a close similarity with the Chalt volcanites of northern Kohistan [12, 14]. 2.3. Saltoro ‘molasse’ The Saltoro ‘molasse’ is a moderately folded sedimentary succession, locally resting with an unconformity, in other
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ing and coarsening upward sequences. This distribution of sedimentary structures suggests an evolution from continental to deeper subaquatic deposition. Near the village of Charasa (figure 1), the red-green shales and sandstones are interbedded with thick (~50 m) porphyritic andesites that show phenocrysts of plagioclase, hornblende and pyroxene. No in situ fossils have been found in the Saltoro ‘molasse’. We recorded a rudist fragment similar to that from the Lower–middle Cretaceous Saltoro formation. Therefore, and since the Saltoro ‘molasse’ rests unconformably on the Saltoro formation, it is younger than the middle Cretaceous. The Saltoro ‘molasse’ has also been intruded by several sets of sills and dykes of intermediate composition indicating that the region was magmatically active after its deposition. Based on its lithological similarity, the occurrence of interbedded porphyritic andesites, and the regional tectonic setting, we suggest that the Saltoro ‘molasse’ be correlated with the Upper Cretaceous to Eocene Purit-Drosh-Reshun formation of Northern Kohistan [13, 17], and thus is distinctly older than the Mio-Pliocene molasse of the Indus Group. 2.4. Ophiolitic melange
Figure 6. Tectonostratigraphic column of the Saltoro formation and associated sequences near the village of Khalsar in the NubraShyok valley.
places along a thrust contact on the Shyok volcanites and on the Saltoro formation. It consists of intensely cleaved, red and green shales, siltstones, medium- to thick-bedded, compact sandstones and polymictic conglomerates and breccias. The clasts include serpentinite, shale, mudstone, acidic to basic volcanics, cherty limestone, marble, and granite embedded in a ferrugineous volcanogenic sandstone matrix. The sandstones are lithic to feldspathic, poorly sorted and contain rip-up clasts of red shale. At the base, the shales and compact sandstones locally exhibit desiccation cracks (figure 8) and rain-drop imprints (figure 9) indicating a subaerial, alluvial environment of deposition. Higher up, sedimentary structures include graded bedding, parallel lamination, cross lamination and ripple marks with thicken-
To the south the Ophiolitic melange is in tectonic contact with the Shyok volcanites (figures 1 and 2); the northern boundary with the Karakoram batholith is intrusive and in places tectonic (figures 1 and 3). The Ophiolitic melange consists dominantly of schistose volcanic greenstones and black slates with blocks of recrystallized limestone, marble, calcschist, quartzite, siltstone, medium to coarse-grained graded sandstone, polymictic meta-conglomerate, matrixsupported pebbly mudstone and serpentinite (figures 10 and 11). The blocks of polymictic meta-conglomerate have clasts of serpentinite, slate, phyllite, altered lavas, recrystallised limestone and quartzite, suggesting that erosion and deposition alternated with deformation. All rock units show a strong cleavage parallel to the regional strike (NW–SE). The various blocks of sandstone and purple shale show a wealth of different sedimentary structures e.g. parallel lamination, graded bedding, ripple mark, rain-drop imprints, desiccation cracks and some trace fossils. This indicates that the sandstone-shale blocks come from different formations. A similar melange has been reported from the Northern suture zone in Kohistan [17]; therefore, we suggest that the Shyok suture and Northern suture melange zone are equivalents. 2.5. Tirit granitoids Several granitic plutons are exposed immediately south of the Shyok suture (figures 1, 2 and 5). We call these WNW–ESE-aligned plutons collectively the Tirit granitoids. They consist of weakly deformed medium- to coarse-
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Figure 7. (Top) Volcanogenic turbidite of the Saltoro formation with characteristic convolute lamination. Figure 8. (Middle) Red-green shale and compact sandstone of the Saltoro molasse exhibiting well-preserved desiccation cracks. Figure 9. (Bottom) Compact volcanogenic sandstone of the Saltoro molasse showing very well-preserved rain-drop imprints.
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Figure 10. Lithostratigraphy of Saltoro molasse and Shyok ophiolitic melange as seen near the village of Sumur in the NubraShyok valley.
grained rocks, subleucocratic to mesocratic, relatively rich in ferromagnesian minerals and compositionally ranging from granodiorite-tonalite to gabbrodiorite. In several places they have been intruded by vertical, undeformed, NW–SEtrending doleritic and aplitic dykes. The dykes show no evidence of chilling against the walls of granitoids which may indicate that the host rocks were still hot when the dykes intruded. The granitoids are more basic at their margins than in the core and show hybridization and chilled margins along the intrusive contacts with the Shyok volcanites. Fineto medium-grained mafic xenoliths are common and range from a few mm to 50 cm in diameter. In places metasedimentary enclaves are present. The Tirit granitoids consist of plagioclase (oligoclaseandesine), K-feldspar, quartz, and mafic minerals. Plagio-
Figure 11. Schematic diagram of lithological composition of the Shyok ophiolitic melange near the village of Tegar in the NubraShyok valley.
clase laths are euhedral and enclosed within subhedral grains of K-feldspar and quartz. Most plagioclase crystals contain secondary sericite and epidote. Hornblende is abundant in diorites. The granodiorites display graphic intergrowth between quartz and feldspar, and plagioclase laths exhibit oscillatory zoning. Biotite is partly altered to green chlorite. The tonalites have lower K-feldspar and quartz contents than the granodiorites. Zircon, apatite, opaques and epidote are common accessory minerals. 2.5.1. Geochemistry of the Tirit granitoids Thirty-one samples of Tirit granitoids were analyzed and 10 representative examples are presented in table I. Major,
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Table I. Representative geochemical composition of Tirit granitoids. Rock type S.NO
mzdq, IV R57
dq, IV R2
Major Oxides (wt.%) SiO2 56.39 57.16 TiO2 1.1 0.68 Al2O3 16.11 15.88 Fe2O3(t) 8.25 7.27 MnO 0.12 0.13 MgO 5.85 5.24 CaO 7.03 7.01 Na2O 3.05 3.7 K2O 2.63 2.13 P2O5 0.31 0.24 LOI 0.57 0.64 Total 101.41 100.08 Trace Elements (ppm) Ba 342 358 Ni 18.5 n.d Cu 57.3 n.d Zn 77.9 n.d Ga 16 n.d Pb 4 n.d Th 1.6 n.d Rb 117 n.d U 0.9 n.d Sr 315 n.d Y 20 n.d Zr 117 n.d Nb 6 n.d Rare Earth Elements (ppm) La n.d 29.3 Ce n.d 37.2 Nd n.d 15.7 Sm n.d 3.82 Eu n.d 1.15 Gd n.d 3.09 Dy n.d 2.63 Er n.d 1.61 Yb n.d 1.73 Lu n.d 0.246 CIPW Norm q 4.94 5.23 or 15.54 12.59 ab 25.81 31.13 an 22.5 20.43 di 8.36 10.33 C .... .... hy 17.06 14.33 mt 3.22 2.84 il 2.09 1.29 ap 0.72 0.56 Plagioclase An47 An39 A/CNK 0.67 0.74
mzdq, IV R6
gd, IV R9
ad, IV R3
mzdq, IV R16
gd, IV R28
gd, IV R12
gd, IV R24
gd, IV R19
65.62 0.53 16.54 3.93 0.04 1.59 2.97 4.36 4.07 0.19 0.93 100.77
66.18 0.53 16.25 4.03 0.04 1.52 3.28 4.31 3.62 0.21 0.62 100.59
66.59 0.53 15.96 3.9 0.04 1.47 2.95 4.04 4.11 0.19 0.86 100.64
67.72 0.48 16.15 3.59 0.05 1.23 3.33 4.34 3.71 0.23 0.81 101.64
68.3 0.47 16.1 2.67 0.04 1.12 3.65 4.62 3.37 0.21 0.47 101.02
69.04 0.4 16.13 2.61 0.02 1.06 2.98 5 3.61 0.19 0.41 101.45
69.09 0.44 15.98 2.6 0.03 1.05 3.52 4.84 3.42 0.21 0.43 101.61
70.19 0.39 15.5 2.81 0.05 0.83 2.48 4.17 3.99 0.17 0.52 101.1
468 31 10 0 17 15.2 26 158 8.4 289 34 239 8
441 33 27 0 17 11.5 30 146 7.3 294 31 219 7
524 33 22 1 15 19.5 28 149 7.8 263 31 237 7
551 25 8 2 16 5.3 21 125 6.1 282 31 206 6
467 15 6 n.d 19 8.7 17 81 3.1 309 31 211 5
570 19 7 n.d 17 14.5 20 96 4.3 311 27 182 7
513 15 5 n.d 15 9 16 76 2.7 299 30 200 6
572 18 6 6 15 13.4 19 127 6.7 237 24 173 5
36.8 57.1 24.1 5.07 1.08 3.68 3.5 1.96 2 0.281
34.2 52.6 20.5 4.65 0.88 3.5 3.2 1.85 1.87 0.259
43.6 64.4 25 5.14 0.99 3.7 3.34 1.93 1.75 0.259
23 35.1 15.8 4.04 0.9 3.23 3.17 1.74 1.7 0.237
23.8 42 19.3 4.87 1.06 3.11 3.44 1.99 2.2 0.283
17.6 29.8 13.3 3.25 0.76 2.47 2.42 1.4 1.47 0.197
21.9 38 17.1 4.36 1 2.74 3.12 1.79 1.9 0.25
21.5 32.8 14.1 3.3 0.78 2.43 2.36 1.37 1.4 0.2
15.09 24.05 36.89 13.49 .... 0.02 7.02 1.54 1.01 0.44 An27 0.97
17.24 21.39 36.47 14.3 0.49 .... 6.7 1.57 1.01 0.49 An28 0.96
19.09 24.29 34.19 13.28 0.09 .... 4.31 5.65 1.01 0.44 An28 0.97
18.97 21.93 36.72 13.63 1.15 .... 5.32 1.39 0.91 0.53 An27 0.94
19.5 19.92 39.09 13.24 2.86 .... 3.33 1.04 0.89 0.49 An25 0.9
18.4 21.33 42.31 10.91 2.16 .... 3.52 1.01 0.76 0.44 An20 0.92
19.4 20.21 40.96 11.78 3.53 .... 2.78 1.01 0.84 0.49 An22 0.88
23.58 23.58 35.29 11.19 0.22 .... 4.28 1.09 0.74 0.39 An24 0.99
n.d.= not determined
trace and rare earth elements (REE) were determined by X-ray fluorescence spectrometry (Siemens SRS 3000) and
inductively coupled plasma-atomic emission spectrometry (Jobin Yvon JY-70 plus) at the Geochemical Laboratory of
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Figure 12. An-Ab-Or classification diagram for the Tirit and Karakoram granitoids (plotted on the diagram after [35]). Dark triangles = Tirit granitoids; open circles=Karakoram granitoids.
Figure 13. AFM diagram for the Tirit and Karakoram granitoids. Dark triangles=Tirit granitoids; open circles = Karakoram granitoids.
the Wadia Institute of Himalayan Geology, Dehradun. International rock geo-standards BHVO-1, MBH and DGH were used for calibration. Major elements have been used to determine the CIPW norms. The Tirit granitoids have a wide range of SiO2 content (56.39 to 70.83 wt%, table I). They are quartz-diorite to tonalite, granodiorite and granitic rocks (figure 12). The Al2O3 and CaO contents are generally high (15.3 to 17.08 wt% and 2.48 to 7.07 wt%, respectively), which may be related to the calcic plagioclase composition. In most samples the relative concentration of Na2O exceeds that of
K2O. According to AFM and QBF diagrams (figures 13 and 14) most of the Tirit granitoids reflect a subalkaline trend intermediate between calcalkaline and alkaline. Major element Harker variation diagrams (figure 15) show a marked decrease in MgO with increasing SiO2. Similar trends are shown by Fe2O3 and TiO2. The generally decreasing trends of Al2O3, CaO and P2O5 are ill defined. Both major alkali oxides (Na2O and K2O) increase with increasing values of SiO2, but the data points are more scattered. The co-linear, smooth and coherent variation trends of most major oxides suggests magmatic differentia-
Figure 14. Q-B-F diagram showing the subalkaline to calcalkaline trends with cafemic association for the Tirit granitoids, and cafemic and alumino-cafemic associations for the Karakoram granitoids (plotted on the diagram of [36, 37]. Dark triangles = Tirit granitoids; open circle = Karakoram granitoids. Different rock types presented in the diagram are: gr=granite, ad=adamellite, gd=granodiorite, to=tonalite, sq=quartz syenite, mzq=quartz monzonite, mzdq=quartz monzodiorite, dq=quartz diorite, s=syenite, mz=monzonite, mzgo=monzogabbro, go=gabbro.
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Figure 15. Harker variation diagrams for major oxides. Dark triangles = Tirit granitoids; open circles = Karakoram granitoids.
tion. In general, no compositional gap seems to exist among the Tirit granitoids that appear to be co-magmatic. A wide variation in the trace element concentrations has been measured (table I, figure 16). The granitoids are depleted in Nb (5 to 14 ppm) which suggests arc magmatism. All the samples are rich in Sr (237 to 507 ppm); the tonalite samples have very low contents of Rb (5 to 59 ppm) and very high Sr contents (314 to 507 ppm). This may reflect the higher plagioclase percentage in these rocks. The Rb/Sr values of the Tirit granitoids are low (<1). Similarly, the molar A/CNK value (Aluminous Saturation Index (ASI) of Zen [23]; where molar A/CNK = Al2O3/Na2O+K2O+CaO
ratio) in the Tirit granitoids ranges between 0.67 and 1.04 (table I), which suggests a metaluminous nature. A similar relationship can be deduced from the A/CNK versus SiO2 % diagram (figure 17) and from high normative diopside and corundum values (table I). Our data place the Tirit granitoids in the I-type of the classification of Chappel and White [24] (figure 17). The Y/Nb ratio is > 2, further suggesting island arc magmatism. The trace element versus SiO2 variation diagrams (figure 16) mostly show that the concentrations of Rb, Y, Zr, Zn, Nb, U and Ga decrease systematically with increasing silica contents. The scattering of some trace elements (Ba, Sr, Th) may be due to a heterogeneous
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Figure 16. Harker variation diagrams for trace elements. Dark triangles = Tirit granitoids; open circles = Karakoram granitoids.
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Figure 17. SiO2 versus A/CNK diagram for Tirit and Karakoram granitoids showing metaluminous, I-type and peraluminous, S-type granitoids respectively (plotted on the diagram of [38]). Dark triangles = Tirit granitoids; open circles = Karakoram granitoids.
accumulation of some essential and accessory mineral phases which are rich in these elements [25, 26].
The chondrite normalised REE patterns [27] (figures 18a and b) are similar for most samples. All the samples are
a Figure 18 a and b. Chondrite-normalised REE plots for the Tirit granitoids (normalising values after [27]).
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moderately fractionated in their REE contents {(La/Lu)N = (8.05–18.1)}. Overall the REE patterns show an enrichment and a better fractionation in LREE {(La/Sm)N= 2.92 to 5.47} than HREE {(Gd/Lu)N= (1.35–1.78)} with marked negative Eu anomalies, which indicates feldspar fractionation. The Eu/Eu* values range from 0.66 to 1.02. The chondrite normalised REE patterns further show a flat MREE (Middle Rare Earth Element)-HREE pattern with Gd = 2.43–4.44 × chondrite and Yb = 1.4–2.37 × chondrite. Such a MREE-HREE flat pattern is attributed to garnet in the residue melt [28]. The primitive mantle normalized traceelement patterns (figure 19) show a systematic depletion in Ti, P, Sr, Nb and Ba. This depletion and the Nb versus Y, and Rb versus Y+Nb plots (figure 20) confirm the calcalkaline character of these volcanic arc granitoids. The regional tectonic setting, the nature of occurrence and the composition of the Tirit granitoids are similar to the plutonic suites of northern Kohistan [11, 12, 29], which are located to the south of the Northern suture zone [30]. No radiometric age data are available from the Tirit granitoids. 2.6. Karakoram batholith Figure 19. Primitive mantle normalised trace element plots for the Tirit granitoids (normalised values after [27]).
The Karakoram batholith, which lies immediately north of the Shyok ophiolitic melange (figures 1 and 5) constitutes
Figure 20. Nb-Y and Rb-Y+Nb tectonic discrimination diagrams showing volcanic arc granite (VAG) + syn-collision granite (Syn-COLG) setting for the Tirit and Karakoram granitoids respectively (plotted on the diagram of [39]). Dark triangles = Tirit granitoids; open circle = Karakoram granitoids.
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Table II. Representative geochemical composition of Karakoram granitoids. Rock type S.No.
mzq, IV K20
Major Oxides (wt.%) SiO2 61.22 TiO2 0.76 Al2O3 16.67 Fe2O3(t) 6.11 MnO 0.09 MgO 3.12 CaO 3.87 Na2O 2.76 K2O 4.74 P2O5 0.32 LOI 0.62 Total 100.37 Trace Elements (ppm) Ba 1531 Ni 13.3 Cu 14.7 Zn 85.3 Ga 21.2 Pb 19 Th 16.3 Rb 364 U 3.1 Sr 579 Y 27 Zr 347 Nb 19 Rare Earth Elements (ppm) La 113.1 Ce 221.2 Nd 72.8 Sm 12.2 Eu 2.08 Gd 7.6 Dy 4.18 Er 1.46 Yb 1.51 Lu 0.213 CIPW Norm q 12.74 or 28.01 ab 23.35 an 17.11 di 0.73 C .... hy 12.69 mt 2.38 il 1.44 ap 0.74 Plagioclase An42 A/CNK 1
ad, III K18
gd, III K14
gd, IV K11
gd, II K9
gd, II K6
ad. II K17
gd, I K13
ad. I K4
66.33 0.5 15.96 3.89 0.08 1.82 2.29 2.9 3.75 0.15 2 99.67
66.4 0.5 15.41 4.3 0.08 1.9 3.5 3.14 3.56 0.15 1.8 100.74
67.49 0.45 15.47 3.6 0.06 1.57 3.04 3.2 3.2 0.15 1.7 99.93
68.1 0.45 15.46 3.22 0.05 1.11 2.53 3.56 3.56 0.15 1.04 99.23
68.49 0.33 16.46 2.36 0.03 0.64 2.17 4.3 3.99 0.1 0.7 99.57
69.93 0.29 15.63 2.35 0.04 0.62 2.02 4.3 3.52 0.08 0.8 99.58
72.11 0.23 15.13 1.87 0.03 0.49 1.91 4.47 3.3 0.07 0.8 100.41
72.57 0.2 14.77 1.51 0.04 0.35 1.14 4.28 4.74 0.1 0.7 100.26
1004 19.8 4.9 42.6 16.3 37 27.2 119 3.8 496 21 137 8
893 10.2 5.1 52.3 24.2 11.6 11.8 447 6.1 303 25 172 25
933 11.3 10.1 51.4 17.7 25 23.9 150 5.3 654 28 176 13
841 11.4 10.9 50.6 17.2 23 15.2 139 4.3 430 24 167 12
804 10.5 7.9 45.7 18.9 22 11.8 116 2.2 509 24 161 13
702 9 8.9 50.9 18.3 28 13.4 170 4 343 25 169 18
1129 13.8 5.5 52.1 19.3 40 25.9 181 3.7 511 26 178 10
927 12.3 5.8 57 19 36 24.8 136 3.3 402 21 182 14
62.2 125.3 38.9 6.99 1.17 4.81 3.39 1.37 1.62 0.255
42.4 78.9 28.1 5.54 1.07 4.27 3.4 2.08 1.76 0.351
36.5 62.9 25.7 5.98 1.204 3.89 3.06 1.7 1.49 0.225
39.35 69.3 27.6 6.24 0.995 4.11 3.14 1.62 1.32 0.175
41.9 76.3 28.8 5.86 0.964 3.45 1.97 0.93 0.58 0.133
31.1 60.8 21.2 3.11 0.505 2.08 0.94 n.d 0.243 0.019
33.3 63.3 22.7 4.2 0.963 2.73 1.59 0.71 0.56 0.239
52.2 96.2 35.3 6.53 0.904 3.4 1.82 0.9 0.685 0.104
26.48 22.16 24.54 10.38 3.33 .... 7.67 1.52 0.95 0.35 An30 1.23
22.99 21.04 26.57 16.38 0.39 .... 8.27 1.68 0.95 0.35 An38 1
26.88 18.91 27.08 14.1 1.57 .... 6.81 1.41 0.85 0.35 An34 1.09
25.95 21.04 30.12 11.57 1.51 .... 5.33 1.16 0.85 0.35 An28 1.08
22.04 23.58 36.39 10.11 1.36 .... 3.46 0.87 0.63 0.23 An22 1.07
25.25 20.8 36.39 9.5 1.57 .... 3.45 0.92 0.55 0.19 An21 1.09
28.16 19.5 37.82 9.02 0.9 .... 2.74 0.72 0.44 0.16 An19 1.05
26.29 28.01 36.22 5 0.77 .... 2.11 0.58 0.38 0.23 An12 1.04
the southern part of the eastern Karakoram. The batholith represents a morphologically elevated region to the north of the Nubra-Shyok valleys, extending almost parallel to the
Shyok suture (figures 1 and 3). In several places, the sharp contact between the Shyok suture and the Karakoram batholith is defined by the NW–SE trending Karakoram
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strike-slip fault zone, which is punctuated by hot springs (figures 1 and 5). The southern boundary of the Karakoram batholith is defined by mylonites and a ~ 50-m-wide zone of strongly foliated carbonaceous slates, marbles, metaconglomerates, quartzites, micaschist and gneisses. Near the Ophiolitic melange the Karakoram granitoids are leucocratic, coarse-grained, porphyritic orthogneisses. The gneissic character of these rocks decreases northward and the rocks gradually pass into porphyritic granites and granodiorites. The most common rocks of the Karakoram batholith are weakly to moderately deformed muscoviteand biotite-bearing two-mica granites, hornblende-biotite granitoids and medium- to coarse-grained K-feldspar rich granites, enclosing large xenoliths of metasedimentary and mafic rocks. Compositionally, the Karakoram batholith includes granite to quartz monzonite, granodiorite, and tonalite (figure 12). Aplites, pegmatite dykes, fine-grained quartz-feldspathic veins and dykes of intermediate composition are common. The granitic batholith intruded the Carboniferous–Permian sequence of the Karakoram Tethyan zone to the north [31]. 2.6.1. Geochemistry of the Karakoram batholith Eighteen representative samples of Karakoram granitoids were analysed for major, trace and rare earth elements (REE); nine representative analyses are given in table II. The analytical procedures are the same as for the Tirit granitoids. The Karakoram granitoids have a range of SiO2-content from 61.2 to 72.95 wt% (table II). They further show a wide range in Al2O3 (14.77 to 16.67 wt%) and CaO (1.14 to 4.32 wt%). The major oxide versus SiO2 plots show a systematic decrease in TiO2, Al2O3, Fe2O3(t), MnO, MgO, CaO and P2O5 with increasing values of silica (figure 15). According to the AFM and QBF diagrams (figures 13, 14) the Karakoram granitoids are following both calcalkaline and subalkaline trends. The higher A/CNK values than the Tirit granitoids ranging between 0.96 to 1.23 further suggest both a metaluminous and peraluminous nature (table II, figure 17). According to the SiO2 versus A/CNK diagram (figure 17) the Karakoram granitoids are both I-and S-types. Trace elements versus SiO2 variation diagrams (figure 16) show that Rb, Y, Zr, Sr and Nb systematically decrease with increasing values of silica; Pb shows a good negative correlation. Chondrite-normalised REE patterns (figure 21a, b) suggest that all samples are strongly enriched in Light Rare Earth Elements (LREE) (La = 31.1–113.1 × Chondrite) and depleted in Heavy Rare Earth Elements (HREE) (Yb = 0.56–1.76 × Chondrite). This indicates that the Karakoram granitoids are more LREE enriched than the Tirit granitoids. All the samples are highly fractionated in their total REE contents with a (La/Lu)N ratio of 12.95 to 56.94. Similarly, samples K4 and K20 (table II) from the Karakoram granitoids show a significant enrichment in Rb, Ba, Large Ion Lithophile Elements (LILE) and a strong deple-
tion in High Field Strength Elements (HFSE), which suggests a pattern similar to that of syn-collision granitoids. The primitive mantle normalised trace-element patterns (figure 22) show a systematic depletion in Ti, P, Sr, Nb and Ba. This depletion is typical of a calcalkaline magmatism in a subduction zone environment. The Nb versus Y, and Rb versus Y+Nb diagrams (figure 20) further confirm that the Karakoram granitoids are both volcanic arc granites (VAG) and syn-collision granites (Syn-COLG), hence comparing to the continental arc plutonism of the Chile arc [32]. We have dated three samples of the S-type granite collected from the middle part of the eastern Karakoram batholith by using Rb/Sr isotopic whole rock technique. This S-type granite is 83 ± 9 Ma old with an initial 87Sr/86Sr ratio of 0.7994 ± 0.00023 [33, 34]. This age suggests that collision between Kohistan-Ladakh arc and Karakoram block was active 83 ± 9 Ma ago.
3. Conclusion Similarity exists between the Shyok suture of northern Ladakh and the Northern suture of Kohistan. It is likely that the Kohistan and the Ladakh units evolved as a single tectonic domain during the Cretaceous–Palaeogene. We also support Petterson and Windley’s [11] interpretation and suggest that the Shyok suture is older than the Indus suture and closed sometimes between 100–75 Ma. The accretionary processes in the Karakoram region began prior to the final closure of the Indus suture. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture zone (50–60 Ma) and the formation of the Nanga ParbatHaramosh syntaxis separated Kohistan and Ladakh. The Holocene–Recent dextral offset along the Karakoram fault [2] reshaped and rejuvenated the tectonic structures and the architecture of the entire Karakoram, the Shyok suture and the adjoining Indian plate region. Acknowledgements. We are grateful to the Department of Science and Technology, Govt. of India, New Delhi for providing financial assistance under the sponsored Project No. ESS/CA/A9-32/93. We thank the authorities of the Wadia Institute of Himalayan Geology, Dehradun for extending their support. Sincere thanks to Profs Brian Windley, Maurizio Gaetani, Daniel Bernoulli, J.P. Burg and Drs M.G. Petterson, Wilfried Winkler and two anonymous reviewers for their critical reviews and encouragement on an earlier draft of this manuscript which formed the base of the present paper. Thanks are due to Drs P.P. Khanna, N.K. Saini of XRF and ICP-Labs of WIHG for analysing the granite samples and generation of other geochemical data. Rb/Sr dates originated from PRL Ahmedabad, India, under the guidance of Dr J.R. Trivedi. RU thanks the Federal Commission of Scholarships, Switzerland, for providing a fellowship to carry out research at the Institute of Geology,
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a b Figure 21 a and b. Chondrite-normalised REE plots for the Karakoram granitoids (normalised values after [27]).
ETH Zurich, Switzerland, and to Profs Daniel Bernoulli and J.P. Burg for providing every kind of facility, support and encouragement at ETH Zurich.
Figure 22. Primitive mantle normalised trace element plots for the Karakoram granitoids (normalised values after [27]).
References [1] Beck R.A., 13 others, Stratigraphic evidence for an early collision between northwest India and Asia, Nature, 373 (1995) 55–58. [2] Searle M.P., Geological evidence against large scale preHolocene offsets along the Karakoram fault: Implications for the limited extrusion of the Tibetan plateau, Tectonics, 15 (1996) 171–186. [3] Sinha A.K., Upadhyay R., Tectonics and sedimentation in the passive margin, trench, forearc and backarc areas of the Indus Suture zone in Ladakh and Karakoram: a review, Geodin. Acta, 10 (1997) 1–12. [4] Upadhyay R., Sinha A.K., Tectonic evolution of Himalayan Tethys and subsequent Indian plate subduction along Indus Suture zone, Proc. Indian National Science Academy (INSA), New Delhi, 64 A (1998) 659–683. [5] Gansser A., The great suture zone between Himalaya and Tibet, a preliminary account, Sci. Terre Himalaya CNRS, 268 (1977) 181–192. [6] Thakur V.C., Mishra D.K., Tectonic framework of the Indus and Shyok suture zones in eastern Ladakh, northwest Himalaya, Tectonophysics, 101 (1984) 207–220. [7] Rai H., Geological evidence against the Shyok Palaeo-suture, Ladakh Himalaya, Nature, 297 (1982) 142–144. [8] Brookfield M.E., Reynolds P.H., Late Cretaceous emplacement of Indus suture zone, ophiolitic melange and an Eocene–Oligocene magmatic arc on the northern edge of the Indian plate, Earth Planet. Sci. Lett., 55 (1981) 157–162. [9] Reynolds P.H., Brookfield M.E., Halifax G., McNutt R.H., The age and nature of Mesozoic–Tertiary magmatism across the Indus Suture Zone in Kashmir and Ladakh (NW India and Pakistan), Geol. Rundschau, 72 (1983) 981–1004.
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[10] Srimal N., India-Asia collision: implications from the geology of the eastern Karakoram, Geology, 14 (1986) 523–527. [11] Petterson M.G., Windley B.F., Rb-Sr dating of the Kohistan arc-batholith in the Trans-Himalaya of north Pakistan, and tectonic implications, Earth Planet. Sci. Lett., 74 (1985) 45–57. [12] Petterson M.G., Windley B.F., Changing source regions of magmas and crustal growth in the Trans-Himalayas: evidence from the Chalt volcanics and Kohistan batholith, Kohistan, N. Pakistan, Earth Planet. Sci. Lett., 102 (1991) 326–341. [13] Pudsey C.J., Coward M.P., Luff I.W., Shackleton R.M., Windley B.F., Jan M.Q., Collision zone between the Kohistan arc and the Asian plate NW Pakistan, Trans. Royal. Soc. Edinburgh, 76 (1985) 463–479. [14] Treloar P.J., Petterson M.G., Jan Q.M., Sullivan M.A., A re-evaluation of the stratigraphy and evolution of the Kohistan arc sequence, Pakistan Himalaya: implications for magmatic and tectonic arc-building processes, J. Geol. Soc. London, 153 (1996) 681–693. [15] Khan A.M., Treloar P.J., Khan A.M., Khan T., Qazi S.M., Jan Q.M., Geology of the Chalt-Babusar transect, Kohistan terrane, N. Pakistan: implications for the constitution and thickening of island-arc crust, Journal of Asian Earth Sciences, 16 (1998) 253–268. [16] Treloar P.J., Rex D.C., Guise P.G., Coward M.P., Searle M.P., Windley B.F., Petterson M.G., Jan M.Q., Luff I.F., K-Ar and Ar-Ar geochronology of the Himalayan collision in NW Pakistan: constraints on the timing of suturing, deformation, metamorphism and uplift, Tectonics, 8 (1989) 881–909. [17] Pudsey C.J., The northern suture, Pakistan: margin of a Cretaceous island arc, Geol. Mag., 123 (1986) 405–423. [18] Rai H., Geology of the Nubra valley and its significance on the evolution of the Ladakh Himalaya, in: Thakur V.C., Sharma K.K. (Eds.), Geology of Indus Suture Zone of Ladakh, Wadia Institute of Himalayan Geology, Dehradun, 1983, pp. 79–91. [19] Rai H., The Shyok valley (northern Ladakh, India): An entrapped and compressed marginal oceanic basin, J. Him. Geol., 2 (1991) 1–15. [20] Srimal N., Basu A.R., Kyser T.K., Tectonic inferences from oxygen isotopes in volcano-plutonic complexes of the India-Asia collision zone, NW India, Tectonics, 6 (1987) 261–273. [21] Rakesh Chandra, Tectonic framework and geologic feature of accreting back-arc of Hundri and Sasoma area between Shyok and Nubra river water divide, Ladakh Himalaya and Karakoram (J and K), India, unpublished PhD thesis, Garhwal University Srinagar, India, 1998, 227 p. [22] Srikantia S.V., Ganesan T.M., Wangdus W.C., A note on the tectonic framework and geologic set-up of the Pangong TsoChusul sector, Ladakh Himalaya, J. Geol. Soc. India, 23 (1982) 354–357. [23] Zen E-an, Aluminium enrichment in silicate by fractional crystallization: some mineralogic and petrographic constraints, J. Petrol., 27 (1986) 1095–1117. [24] Chappel B.W., White A.J.R., Two contrasting granite types, Pacific Geology, 8 (1974) 173–174.
[25] Pearce J.A., Norry M.J., Petrogenetic implications of Ti, Zr, Y and Nb variations in volcanic rocks, Contr. Miner. Pet., 69 (1979) 33–47. [26] Barker F., Arth J.G., Generation of Trondjemite-Tonalite liquids and Archean biomodal trondjemite, basalt suites, Geology, 4 (1976) 596–600. [27] Sun S.S., McDonough W.F., Chemical and isotopic systematics of oceanic basalt: implications for mantle composition and processes, Saunders A.D., Norry M.J. (Eds.), Magmatism in the Ocean Basins, Geol. Soc. Spec. Pub., 42 (1989) 313–345. [28] Henderson P., Rare earth element geochemistry, Elsevier Science Pub. Amsterdam, Netherlands, 1984, 510 p. [29] Debon F., Le Fort P., Dautel D., Sonet J., Zimmermann J.L., Granitoids of western Karakoram and northern Kohistan (Pakistan): a composite Mid-Cretaceous to Upper Cenozoic magmatism, Lithos, 20 (1987) 19–24. [30] Coward M.P., Windley B.F., Broughton R.D., Luff I.W., Petterson M.G., Pudsey C.J., Rex D.C., Khan A.M., Collision tectonics in the NW Himalayas, Coward M.P., Ries A.C. (Eds.), Collision Tectonics, Geol. Soc. Spec. Pub., 19 (1986) 203–219. [31] Sinha A.K., Rai H., Upadhyay R., Chandra R., Contribution to the geology of the eastern Karakoram, India, Geol. Soc. America Spec. Paper, 328 (1999) 33–46. [32] Baldwin A.J., Pearce J.A., Discrimination of productive and non-productive porphyritic intrusions in the Chilean Andes, Economic Geology, 77 (1982) 664–674. [33] Sinha A.K., Trivedi J.R., Upadhyay R., Rai H., Chandra R., Geochemical and geochronological studies of granitoids of eastern Karakoram and their impact on the tectonic interpretation, Extended abstract in International Conference on Isotope in the Solar System (11–14 Nov., 1997), PRL Ahmedabad, 1997, 116– 117. [34] Trivedi J.R., Upadhyay R., Chandra R., Rai H., Sinha A.K., Rb/Sr dates from granitoids of eastern Karakoram batholith, India, and their implication in accretion tectonics, Jour. Nepal Geol. Soc., 16 (1997) 29–30. [35] O’ Connor J.T., A classification for quartz-rich igneous rocks based on feldspar ratios, US Geol. Surv. Prof. Paper 525B (1965) B79–B84. [36] Debon F., Le Fort P., A chemical-mineralogical classification of common plutonic rocks and associations, Trans. R. Soc. Edinburgh, 73 (1983) 135–149. [37] Debon F., Le Fort P., A cationic classification of common plutonic rocks and their magmatic association: principles, methods, applications, Bull. Mineral., 111 (1988) 493–510. [38] White A.J.R., Chappell B.W., Ultra-metamorphism and granitoid genesis, Tectonophysics, 43 (1977) 7–22. [39] Pearce J.A., Harris N.B.W., Tindle A.G., Trace element discrimination diagrams for the tectonic interpretation of granitic rocks, J. Petrol., 25 (1984) 956–983.
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Tectonic and magmatic evolution of the eastern Karakoram, India Rajeev Upadhyaya*, Anshu K. Sinhab, Rakesh Chandrac, Hakim Raid a b
d
Institute of Geology, ETH-Zentrum, 8092 Zurich, Switzerland Birbal Sahni Institute of Palaeobotany, 226007 Lucknow, India c Government Degree College, 176215 Dharamshala, India Wadia Institute of Himalayan Geology, 248001 Dehradun, India Received 2 September 1999; accepted 1 October 1999
Abstract – The Shyok suture zone separates the Ladakh terrane to the SW from the Karakoram terrane to the NE. Six tectonic units have been distinguished. From south to north these are: 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram terrane including the Karakoram batholith. Albian–Aptian Orbitolina-bearing limestones and turbidites of the Saltoro formation tectonically overlie high-Mg-tholeiites similar to the tectonically overlying Shyok volcanites. The high-Mg tholeiitic basalts and calcalkaline andesites of the Shyok volcanites show an active margin signature. The Saltoro molasse is an apron-like, moderately folded association of redgreen shales and sandstones that are interbedded with ~ 50 m porphyritic andesite. Desiccation cracks and rain-drop imprints indicate deposition in a subaerial fluvial environment. Rudist fragments from a polygenic conglomerate of the Saltoro molasse document a post-Middle Cretaceous age. The calcalkaline andesites of the Shyok volcanites are intruded by the Tirit granitoids, which are located immediately south of the Ophiolitic melange and belong to a weakly deformed trondhjemite-tonalite-granodiorite-granite suite. These granitoids are subalkaline, I-type and were emplaced in a volcanic arc setting. The subalkaline to calcalkaline granitoids of the Karakoram batholith are I-and S-type granitoid. The I-type granitoids represent a typical calcalkaline magmatism of a subduction zone environment whereas the S-type granitoids are crustderived, anatectic peraluminous granites. New data suggest that the volcano-plutonic and sedimentary successions of the Shyok suture zone exposed in northern Ladakh are equivalent to the successions exposed along the Northern suture in Kohistan. It is likely that the Kohistan and Ladakh blocks evolved as one single tectonic domain during the Cretaceous–Palaeogene. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture (50–60 Ma) together with tectonic activity along the Nanga ParbatHaramosh divided Kohistan and Ladakh into two arealy distinct magmatic arc terranes. The activity and a dextral offset along the
* Correspondence and reprints: [email protected]
Karakoram fault (Holocene–Recent) disrupted the original tectonic relationships. © 1999 Éditions scientifiques et médicales Elsevier SAS subduction / magmatism / collision / accretion / Karakoram
1. Introduction Closure of the Neo-Tethys ocean occurred along the Indus-Tsangpo suture zone between 60 and 50 Ma [1–3]. The Indus suture zone is marked by obducted remnants of the former Neo-Tethyan oceanic lithosphere along with Triassic to Eocene marine sediments [3, 4]. To the north of the Indus suture zone, in Ladakh, another important tectonic zone is documented by the rocks of the Shyok suture zone and the Karakoram batholith (figures 1 and 2). The Shyok suture is interpreted as an oceanic suture [5] or the relic of a back-arc basin [6]. Rai [7] argued against the existence of a subduction zone along the Shyok suture. In his interpretation, the Indus and Shyok zones are segments of one single suture which has been cut by the Ladakh batholith. Brookfield and Reynolds [8] and Reynolds et al. [9] suggested that the Shyok suture did not close before the Miocene and therefore considered it to be younger than the Indus suture. Srimal [10], however, suggested that the tectonic units accompanying the Shyok ophiolites were part of a collage between the northern margin of India and the southern margin of the Asian plate. Significant contributions have been made to the understanding of the tectonic evolution of the western continuation of the Shyok suture zone, the Northern suture exposed in northern Pakistan [11–15] (figure 1). It was postulated that the Northern suture resulted from collision between the Kohistan island arc and the Asian margin
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Figure 1. Geological map of the Shyok suture zone in Nubra-Shyok valley, Saltoro hills, northern Ladakh, India. K.K. Fault refers to the Karakoram fault; A–B = section of figure 2.
between 100 and 75 Ma [16]. A minimum age for the closure of the Northern suture is given by the northeast–
southwest trending Jutal-Nomal diorite dykes that yielded a Ar/40Ar age of 75 Ma [11] and cut across structures related
39
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Figure 2. Geological cross-section along line A–B on figure 2. KF refers to the Karakoram fault. Legend as on map (figure 1).
to the Suture. A maximum age is indicated by the youngest marine sediments, which are Aptian–Albian in age [17]. No detailed study, however, has been made to reconstruct the tectonic evolution of the Shyok suture in northern Ladakh [3]. Here we report new field observations on stratigraphy, sedimentology, and geochemical data of plutonic rocks of the Shyok suture zone. We outline its geology along a transect in the Nubra-Shyok river valleys. We describe a thick sedimentary-volcanic succession yielding Middle Cretaceous foraminifera. We also discuss the tectonic implications for regional correlation and comparison with the better investigated areas of northern Kohistan.
2. Geological framework of the Shyok suture zone in eastern Karakoram The rocks of the Shyok suture zone, trending northwest– southeast (figure 1) across the Nubra-Shyok valley, occur in intensely deformed tectonic slices between the Ladakh
batholith, to the southwest, and Karakoram batholith (figure 3) to the northeast. These tectonic slices comprise a variety of sedimentary, volcanic and plutonic rocks. Across a traverse through the Shyok-Nubra river valleys and the adjoining part of the Karakoram terrane, the major tectonic slices described in this paper are made up of the following rock units, from south to north i.e., from the structural bottom to top: 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram batholith. 2.1. Saltoro Formation The lowermost imbricate of the Shyok suture zone consists of the Saltoro formation (the Saltoro flysch [18, 19]) (figures 1 and 2). This formation begins at its base with a very low grade, gently folded calcareous succession which, to the south, is separated by a steeply dipping thrust from the Upper Cretaceous to Eocene Khardung formation of the Indus suture zone (figures 4 and 5) [20]. The lower part of
Figure 3. Panoramic view of the Nubra river valley showing the tectonic juxtaposition of the Shyok ophiolitic melange and the Karakoram batholith. View looking towards NE of the Charasa in figure 1.
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Figure 4. Field sketch cross-section across the Shyok suture zone near the village of Khalsar in figure 2. Tonalite refers to the Tirit granitoids.
the Saltoro formation is well exposed to the south of the village of Khalsar (figure 1) where it consists of thinly and mostly even-bedded, highly fissile and cleaved slates, phyllites, siltstones and intercalations of thinly to mediumbedded grey limestones and marbles. We have found a bryozoan fauna from a 5-m-thick, thinly bedded recrystallised limestone horizon (figures 1 and 6). These bryozoans were identified as Cheilostomata of possible Late Jurassic age (identification by Prof. A.K. Guha, I.I.T., Kharagpur). Other fossils are recrystallised echinoids, foraminifera and crinoid stems of undeterminable age. Near Horo Sosten (figure 1), we have identified a 150–200-m-thick limestone succession tectonically resting on volcanic rocks identified as high-Mg tholeiitic basalts of island arc affinity [21]. The partly recrystallised limestone contains abundant bivalves and other molluscs. Thin-section examination yielded rudists and a rich foraminiferal assemblage including Orbitolina discoidea, Palorbitolina and Texularia of Early to Middle Cretaceous age. This limestone horizon may be correlated with the Aptian–Albian Yasin Group in northern Kohistan [17] and with a volcano-sedimentary succession reported farther southeast near Pangong Tso lake (~150 km southeast of Khalsar, [22]). To the northeast, the fossilifer-
ous limestone is overlain with a tectonic contact by a ~1 500-m-thick volcanogenic sequence that is more calcareous near its base. The turbidites are interbedded with thinto medium-bedded black pyritic shales, grey to green fissile slate, siltstone and medium- to thick-bedded, medium- to coarse-grained compact sandstone to siltstone. The sedimentary structures include flute casts, graded bedding, parallel, wavy and convolute laminations (figure 7) and currentripples. 2.2. Shyok volcanite Under the heading of Saltoro volcanite we reunite different occurrences of volcanites of which we do not know the original palaeogeographic position. Sporadic outcrops occur below the Saltoro formation near Shukur (figure 1). Southeast of Diskit the Saltoro formation is tectonically overlain along a steeply dipping thrust by chlorite schists, basic volcanites and cherts of the Shyok volcanite (figures 1, 2 and 4). West of the Karakoram fault they also occur tectonically sandwiched between the underlying Saltoro molasse and the Ophiolitic melange, above. East of the Karakoram fault, they crop out between the villages of Panamik and Tirit and
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Figure 5. Detailed field sketch across the Nubra-Shyok valley showing the different tectonic imbricates of the Shyok suture zone near the Nubra and Shyok river confluence point in figure 2. Tonalite = Tirit granitoids; K.K. Fault = Karakoram fault.
near the confluence of the Nubra-Shyok rivers. These volcanites are the most widespread lithological unit in the Nubra-Shyok valley (figure 1). Rai [18] estimated that they are up to 4 km thick, consisting of a heterogeneous sequence of basalts and andesites with ignimbrites. The detailed geochemistry of the Shyok volcanites is beyond the scope of this paper, but preliminary geochemical data suggest that they may include different units. The volcanic rocks exposed near Shukur are fine- to mediumgrained, massive and highly fractured high-Mg tholeiites with rare vesicles; however between the villages of Kuri and Tirit and near the Nubra-Shyok confluence they are basaltic andesites. These andesites are medium-grained, strongly epidotised and agglomeratic in texture; vesicles are well developed, vary from a few mm to 1 cm in diameter and are filled with calcite, zeolites and chlorite. The basaltic andesites are also cut by hydrothermal veins rich in sulfide mineralisations, mainly chalcopyrite and flaky hematite. The preliminary geochemical data further suggest that, like the
high-Mg tholeiites the basaltic andesites are calcalkaline volcanites. No radiometric data are available. Since the Shyok volcanites are separated by faults from the surrounding rocks, we can only speculate about their age. The Lower–middle Cretaceous Orbitolina-bearing limestone from the Shukur village may provide a time constraint because we think that the tectonic contact near Shukur is not a major fault and, therefore, the volcanites may well be the base to the overlying limestones. Based on field relationships, regional distribution, similarity in composition and geochemical characteristics, it is suggested that the Shyok volcanites have a close similarity with the Chalt volcanites of northern Kohistan [12, 14]. 2.3. Saltoro ‘molasse’ The Saltoro ‘molasse’ is a moderately folded sedimentary succession, locally resting with an unconformity, in other
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ing and coarsening upward sequences. This distribution of sedimentary structures suggests an evolution from continental to deeper subaquatic deposition. Near the village of Charasa (figure 1), the red-green shales and sandstones are interbedded with thick (~50 m) porphyritic andesites that show phenocrysts of plagioclase, hornblende and pyroxene. No in situ fossils have been found in the Saltoro ‘molasse’. We recorded a rudist fragment similar to that from the Lower–middle Cretaceous Saltoro formation. Therefore, and since the Saltoro ‘molasse’ rests unconformably on the Saltoro formation, it is younger than the middle Cretaceous. The Saltoro ‘molasse’ has also been intruded by several sets of sills and dykes of intermediate composition indicating that the region was magmatically active after its deposition. Based on its lithological similarity, the occurrence of interbedded porphyritic andesites, and the regional tectonic setting, we suggest that the Saltoro ‘molasse’ be correlated with the Upper Cretaceous to Eocene Purit-Drosh-Reshun formation of Northern Kohistan [13, 17], and thus is distinctly older than the Mio-Pliocene molasse of the Indus Group. 2.4. Ophiolitic melange
Figure 6. Tectonostratigraphic column of the Saltoro formation and associated sequences near the village of Khalsar in the NubraShyok valley.
places along a thrust contact on the Shyok volcanites and on the Saltoro formation. It consists of intensely cleaved, red and green shales, siltstones, medium- to thick-bedded, compact sandstones and polymictic conglomerates and breccias. The clasts include serpentinite, shale, mudstone, acidic to basic volcanics, cherty limestone, marble, and granite embedded in a ferrugineous volcanogenic sandstone matrix. The sandstones are lithic to feldspathic, poorly sorted and contain rip-up clasts of red shale. At the base, the shales and compact sandstones locally exhibit desiccation cracks (figure 8) and rain-drop imprints (figure 9) indicating a subaerial, alluvial environment of deposition. Higher up, sedimentary structures include graded bedding, parallel lamination, cross lamination and ripple marks with thicken-
To the south the Ophiolitic melange is in tectonic contact with the Shyok volcanites (figures 1 and 2); the northern boundary with the Karakoram batholith is intrusive and in places tectonic (figures 1 and 3). The Ophiolitic melange consists dominantly of schistose volcanic greenstones and black slates with blocks of recrystallized limestone, marble, calcschist, quartzite, siltstone, medium to coarse-grained graded sandstone, polymictic meta-conglomerate, matrixsupported pebbly mudstone and serpentinite (figures 10 and 11). The blocks of polymictic meta-conglomerate have clasts of serpentinite, slate, phyllite, altered lavas, recrystallised limestone and quartzite, suggesting that erosion and deposition alternated with deformation. All rock units show a strong cleavage parallel to the regional strike (NW–SE). The various blocks of sandstone and purple shale show a wealth of different sedimentary structures e.g. parallel lamination, graded bedding, ripple mark, rain-drop imprints, desiccation cracks and some trace fossils. This indicates that the sandstone-shale blocks come from different formations. A similar melange has been reported from the Northern suture zone in Kohistan [17]; therefore, we suggest that the Shyok suture and Northern suture melange zone are equivalents. 2.5. Tirit granitoids Several granitic plutons are exposed immediately south of the Shyok suture (figures 1, 2 and 5). We call these WNW–ESE-aligned plutons collectively the Tirit granitoids. They consist of weakly deformed medium- to coarse-
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Figure 7. (Top) Volcanogenic turbidite of the Saltoro formation with characteristic convolute lamination. Figure 8. (Middle) Red-green shale and compact sandstone of the Saltoro molasse exhibiting well-preserved desiccation cracks. Figure 9. (Bottom) Compact volcanogenic sandstone of the Saltoro molasse showing very well-preserved rain-drop imprints.
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Figure 10. Lithostratigraphy of Saltoro molasse and Shyok ophiolitic melange as seen near the village of Sumur in the NubraShyok valley.
grained rocks, subleucocratic to mesocratic, relatively rich in ferromagnesian minerals and compositionally ranging from granodiorite-tonalite to gabbrodiorite. In several places they have been intruded by vertical, undeformed, NW–SEtrending doleritic and aplitic dykes. The dykes show no evidence of chilling against the walls of granitoids which may indicate that the host rocks were still hot when the dykes intruded. The granitoids are more basic at their margins than in the core and show hybridization and chilled margins along the intrusive contacts with the Shyok volcanites. Fineto medium-grained mafic xenoliths are common and range from a few mm to 50 cm in diameter. In places metasedimentary enclaves are present. The Tirit granitoids consist of plagioclase (oligoclaseandesine), K-feldspar, quartz, and mafic minerals. Plagio-
Figure 11. Schematic diagram of lithological composition of the Shyok ophiolitic melange near the village of Tegar in the NubraShyok valley.
clase laths are euhedral and enclosed within subhedral grains of K-feldspar and quartz. Most plagioclase crystals contain secondary sericite and epidote. Hornblende is abundant in diorites. The granodiorites display graphic intergrowth between quartz and feldspar, and plagioclase laths exhibit oscillatory zoning. Biotite is partly altered to green chlorite. The tonalites have lower K-feldspar and quartz contents than the granodiorites. Zircon, apatite, opaques and epidote are common accessory minerals. 2.5.1. Geochemistry of the Tirit granitoids Thirty-one samples of Tirit granitoids were analyzed and 10 representative examples are presented in table I. Major,
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Table I. Representative geochemical composition of Tirit granitoids. Rock type S.NO
mzdq, IV R57
dq, IV R2
Major Oxides (wt.%) SiO2 56.39 57.16 TiO2 1.1 0.68 Al2O3 16.11 15.88 Fe2O3(t) 8.25 7.27 MnO 0.12 0.13 MgO 5.85 5.24 CaO 7.03 7.01 Na2O 3.05 3.7 K2O 2.63 2.13 P2O5 0.31 0.24 LOI 0.57 0.64 Total 101.41 100.08 Trace Elements (ppm) Ba 342 358 Ni 18.5 n.d Cu 57.3 n.d Zn 77.9 n.d Ga 16 n.d Pb 4 n.d Th 1.6 n.d Rb 117 n.d U 0.9 n.d Sr 315 n.d Y 20 n.d Zr 117 n.d Nb 6 n.d Rare Earth Elements (ppm) La n.d 29.3 Ce n.d 37.2 Nd n.d 15.7 Sm n.d 3.82 Eu n.d 1.15 Gd n.d 3.09 Dy n.d 2.63 Er n.d 1.61 Yb n.d 1.73 Lu n.d 0.246 CIPW Norm q 4.94 5.23 or 15.54 12.59 ab 25.81 31.13 an 22.5 20.43 di 8.36 10.33 C .... .... hy 17.06 14.33 mt 3.22 2.84 il 2.09 1.29 ap 0.72 0.56 Plagioclase An47 An39 A/CNK 0.67 0.74
mzdq, IV R6
gd, IV R9
ad, IV R3
mzdq, IV R16
gd, IV R28
gd, IV R12
gd, IV R24
gd, IV R19
65.62 0.53 16.54 3.93 0.04 1.59 2.97 4.36 4.07 0.19 0.93 100.77
66.18 0.53 16.25 4.03 0.04 1.52 3.28 4.31 3.62 0.21 0.62 100.59
66.59 0.53 15.96 3.9 0.04 1.47 2.95 4.04 4.11 0.19 0.86 100.64
67.72 0.48 16.15 3.59 0.05 1.23 3.33 4.34 3.71 0.23 0.81 101.64
68.3 0.47 16.1 2.67 0.04 1.12 3.65 4.62 3.37 0.21 0.47 101.02
69.04 0.4 16.13 2.61 0.02 1.06 2.98 5 3.61 0.19 0.41 101.45
69.09 0.44 15.98 2.6 0.03 1.05 3.52 4.84 3.42 0.21 0.43 101.61
70.19 0.39 15.5 2.81 0.05 0.83 2.48 4.17 3.99 0.17 0.52 101.1
468 31 10 0 17 15.2 26 158 8.4 289 34 239 8
441 33 27 0 17 11.5 30 146 7.3 294 31 219 7
524 33 22 1 15 19.5 28 149 7.8 263 31 237 7
551 25 8 2 16 5.3 21 125 6.1 282 31 206 6
467 15 6 n.d 19 8.7 17 81 3.1 309 31 211 5
570 19 7 n.d 17 14.5 20 96 4.3 311 27 182 7
513 15 5 n.d 15 9 16 76 2.7 299 30 200 6
572 18 6 6 15 13.4 19 127 6.7 237 24 173 5
36.8 57.1 24.1 5.07 1.08 3.68 3.5 1.96 2 0.281
34.2 52.6 20.5 4.65 0.88 3.5 3.2 1.85 1.87 0.259
43.6 64.4 25 5.14 0.99 3.7 3.34 1.93 1.75 0.259
23 35.1 15.8 4.04 0.9 3.23 3.17 1.74 1.7 0.237
23.8 42 19.3 4.87 1.06 3.11 3.44 1.99 2.2 0.283
17.6 29.8 13.3 3.25 0.76 2.47 2.42 1.4 1.47 0.197
21.9 38 17.1 4.36 1 2.74 3.12 1.79 1.9 0.25
21.5 32.8 14.1 3.3 0.78 2.43 2.36 1.37 1.4 0.2
15.09 24.05 36.89 13.49 .... 0.02 7.02 1.54 1.01 0.44 An27 0.97
17.24 21.39 36.47 14.3 0.49 .... 6.7 1.57 1.01 0.49 An28 0.96
19.09 24.29 34.19 13.28 0.09 .... 4.31 5.65 1.01 0.44 An28 0.97
18.97 21.93 36.72 13.63 1.15 .... 5.32 1.39 0.91 0.53 An27 0.94
19.5 19.92 39.09 13.24 2.86 .... 3.33 1.04 0.89 0.49 An25 0.9
18.4 21.33 42.31 10.91 2.16 .... 3.52 1.01 0.76 0.44 An20 0.92
19.4 20.21 40.96 11.78 3.53 .... 2.78 1.01 0.84 0.49 An22 0.88
23.58 23.58 35.29 11.19 0.22 .... 4.28 1.09 0.74 0.39 An24 0.99
n.d.= not determined
trace and rare earth elements (REE) were determined by X-ray fluorescence spectrometry (Siemens SRS 3000) and
inductively coupled plasma-atomic emission spectrometry (Jobin Yvon JY-70 plus) at the Geochemical Laboratory of
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Figure 12. An-Ab-Or classification diagram for the Tirit and Karakoram granitoids (plotted on the diagram after [35]). Dark triangles = Tirit granitoids; open circles=Karakoram granitoids.
Figure 13. AFM diagram for the Tirit and Karakoram granitoids. Dark triangles=Tirit granitoids; open circles = Karakoram granitoids.
the Wadia Institute of Himalayan Geology, Dehradun. International rock geo-standards BHVO-1, MBH and DGH were used for calibration. Major elements have been used to determine the CIPW norms. The Tirit granitoids have a wide range of SiO2 content (56.39 to 70.83 wt%, table I). They are quartz-diorite to tonalite, granodiorite and granitic rocks (figure 12). The Al2O3 and CaO contents are generally high (15.3 to 17.08 wt% and 2.48 to 7.07 wt%, respectively), which may be related to the calcic plagioclase composition. In most samples the relative concentration of Na2O exceeds that of
K2O. According to AFM and QBF diagrams (figures 13 and 14) most of the Tirit granitoids reflect a subalkaline trend intermediate between calcalkaline and alkaline. Major element Harker variation diagrams (figure 15) show a marked decrease in MgO with increasing SiO2. Similar trends are shown by Fe2O3 and TiO2. The generally decreasing trends of Al2O3, CaO and P2O5 are ill defined. Both major alkali oxides (Na2O and K2O) increase with increasing values of SiO2, but the data points are more scattered. The co-linear, smooth and coherent variation trends of most major oxides suggests magmatic differentia-
Figure 14. Q-B-F diagram showing the subalkaline to calcalkaline trends with cafemic association for the Tirit granitoids, and cafemic and alumino-cafemic associations for the Karakoram granitoids (plotted on the diagram of [36, 37]. Dark triangles = Tirit granitoids; open circle = Karakoram granitoids. Different rock types presented in the diagram are: gr=granite, ad=adamellite, gd=granodiorite, to=tonalite, sq=quartz syenite, mzq=quartz monzonite, mzdq=quartz monzodiorite, dq=quartz diorite, s=syenite, mz=monzonite, mzgo=monzogabbro, go=gabbro.
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Figure 15. Harker variation diagrams for major oxides. Dark triangles = Tirit granitoids; open circles = Karakoram granitoids.
tion. In general, no compositional gap seems to exist among the Tirit granitoids that appear to be co-magmatic. A wide variation in the trace element concentrations has been measured (table I, figure 16). The granitoids are depleted in Nb (5 to 14 ppm) which suggests arc magmatism. All the samples are rich in Sr (237 to 507 ppm); the tonalite samples have very low contents of Rb (5 to 59 ppm) and very high Sr contents (314 to 507 ppm). This may reflect the higher plagioclase percentage in these rocks. The Rb/Sr values of the Tirit granitoids are low (<1). Similarly, the molar A/CNK value (Aluminous Saturation Index (ASI) of Zen [23]; where molar A/CNK = Al2O3/Na2O+K2O+CaO
ratio) in the Tirit granitoids ranges between 0.67 and 1.04 (table I), which suggests a metaluminous nature. A similar relationship can be deduced from the A/CNK versus SiO2 % diagram (figure 17) and from high normative diopside and corundum values (table I). Our data place the Tirit granitoids in the I-type of the classification of Chappel and White [24] (figure 17). The Y/Nb ratio is > 2, further suggesting island arc magmatism. The trace element versus SiO2 variation diagrams (figure 16) mostly show that the concentrations of Rb, Y, Zr, Zn, Nb, U and Ga decrease systematically with increasing silica contents. The scattering of some trace elements (Ba, Sr, Th) may be due to a heterogeneous
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Figure 16. Harker variation diagrams for trace elements. Dark triangles = Tirit granitoids; open circles = Karakoram granitoids.
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Figure 17. SiO2 versus A/CNK diagram for Tirit and Karakoram granitoids showing metaluminous, I-type and peraluminous, S-type granitoids respectively (plotted on the diagram of [38]). Dark triangles = Tirit granitoids; open circles = Karakoram granitoids.
accumulation of some essential and accessory mineral phases which are rich in these elements [25, 26].
The chondrite normalised REE patterns [27] (figures 18a and b) are similar for most samples. All the samples are
a Figure 18 a and b. Chondrite-normalised REE plots for the Tirit granitoids (normalising values after [27]).
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moderately fractionated in their REE contents {(La/Lu)N = (8.05–18.1)}. Overall the REE patterns show an enrichment and a better fractionation in LREE {(La/Sm)N= 2.92 to 5.47} than HREE {(Gd/Lu)N= (1.35–1.78)} with marked negative Eu anomalies, which indicates feldspar fractionation. The Eu/Eu* values range from 0.66 to 1.02. The chondrite normalised REE patterns further show a flat MREE (Middle Rare Earth Element)-HREE pattern with Gd = 2.43–4.44 × chondrite and Yb = 1.4–2.37 × chondrite. Such a MREE-HREE flat pattern is attributed to garnet in the residue melt [28]. The primitive mantle normalized traceelement patterns (figure 19) show a systematic depletion in Ti, P, Sr, Nb and Ba. This depletion and the Nb versus Y, and Rb versus Y+Nb plots (figure 20) confirm the calcalkaline character of these volcanic arc granitoids. The regional tectonic setting, the nature of occurrence and the composition of the Tirit granitoids are similar to the plutonic suites of northern Kohistan [11, 12, 29], which are located to the south of the Northern suture zone [30]. No radiometric age data are available from the Tirit granitoids. 2.6. Karakoram batholith Figure 19. Primitive mantle normalised trace element plots for the Tirit granitoids (normalised values after [27]).
The Karakoram batholith, which lies immediately north of the Shyok ophiolitic melange (figures 1 and 5) constitutes
Figure 20. Nb-Y and Rb-Y+Nb tectonic discrimination diagrams showing volcanic arc granite (VAG) + syn-collision granite (Syn-COLG) setting for the Tirit and Karakoram granitoids respectively (plotted on the diagram of [39]). Dark triangles = Tirit granitoids; open circle = Karakoram granitoids.
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Table II. Representative geochemical composition of Karakoram granitoids. Rock type S.No.
mzq, IV K20
Major Oxides (wt.%) SiO2 61.22 TiO2 0.76 Al2O3 16.67 Fe2O3(t) 6.11 MnO 0.09 MgO 3.12 CaO 3.87 Na2O 2.76 K2O 4.74 P2O5 0.32 LOI 0.62 Total 100.37 Trace Elements (ppm) Ba 1531 Ni 13.3 Cu 14.7 Zn 85.3 Ga 21.2 Pb 19 Th 16.3 Rb 364 U 3.1 Sr 579 Y 27 Zr 347 Nb 19 Rare Earth Elements (ppm) La 113.1 Ce 221.2 Nd 72.8 Sm 12.2 Eu 2.08 Gd 7.6 Dy 4.18 Er 1.46 Yb 1.51 Lu 0.213 CIPW Norm q 12.74 or 28.01 ab 23.35 an 17.11 di 0.73 C .... hy 12.69 mt 2.38 il 1.44 ap 0.74 Plagioclase An42 A/CNK 1
ad, III K18
gd, III K14
gd, IV K11
gd, II K9
gd, II K6
ad. II K17
gd, I K13
ad. I K4
66.33 0.5 15.96 3.89 0.08 1.82 2.29 2.9 3.75 0.15 2 99.67
66.4 0.5 15.41 4.3 0.08 1.9 3.5 3.14 3.56 0.15 1.8 100.74
67.49 0.45 15.47 3.6 0.06 1.57 3.04 3.2 3.2 0.15 1.7 99.93
68.1 0.45 15.46 3.22 0.05 1.11 2.53 3.56 3.56 0.15 1.04 99.23
68.49 0.33 16.46 2.36 0.03 0.64 2.17 4.3 3.99 0.1 0.7 99.57
69.93 0.29 15.63 2.35 0.04 0.62 2.02 4.3 3.52 0.08 0.8 99.58
72.11 0.23 15.13 1.87 0.03 0.49 1.91 4.47 3.3 0.07 0.8 100.41
72.57 0.2 14.77 1.51 0.04 0.35 1.14 4.28 4.74 0.1 0.7 100.26
1004 19.8 4.9 42.6 16.3 37 27.2 119 3.8 496 21 137 8
893 10.2 5.1 52.3 24.2 11.6 11.8 447 6.1 303 25 172 25
933 11.3 10.1 51.4 17.7 25 23.9 150 5.3 654 28 176 13
841 11.4 10.9 50.6 17.2 23 15.2 139 4.3 430 24 167 12
804 10.5 7.9 45.7 18.9 22 11.8 116 2.2 509 24 161 13
702 9 8.9 50.9 18.3 28 13.4 170 4 343 25 169 18
1129 13.8 5.5 52.1 19.3 40 25.9 181 3.7 511 26 178 10
927 12.3 5.8 57 19 36 24.8 136 3.3 402 21 182 14
62.2 125.3 38.9 6.99 1.17 4.81 3.39 1.37 1.62 0.255
42.4 78.9 28.1 5.54 1.07 4.27 3.4 2.08 1.76 0.351
36.5 62.9 25.7 5.98 1.204 3.89 3.06 1.7 1.49 0.225
39.35 69.3 27.6 6.24 0.995 4.11 3.14 1.62 1.32 0.175
41.9 76.3 28.8 5.86 0.964 3.45 1.97 0.93 0.58 0.133
31.1 60.8 21.2 3.11 0.505 2.08 0.94 n.d 0.243 0.019
33.3 63.3 22.7 4.2 0.963 2.73 1.59 0.71 0.56 0.239
52.2 96.2 35.3 6.53 0.904 3.4 1.82 0.9 0.685 0.104
26.48 22.16 24.54 10.38 3.33 .... 7.67 1.52 0.95 0.35 An30 1.23
22.99 21.04 26.57 16.38 0.39 .... 8.27 1.68 0.95 0.35 An38 1
26.88 18.91 27.08 14.1 1.57 .... 6.81 1.41 0.85 0.35 An34 1.09
25.95 21.04 30.12 11.57 1.51 .... 5.33 1.16 0.85 0.35 An28 1.08
22.04 23.58 36.39 10.11 1.36 .... 3.46 0.87 0.63 0.23 An22 1.07
25.25 20.8 36.39 9.5 1.57 .... 3.45 0.92 0.55 0.19 An21 1.09
28.16 19.5 37.82 9.02 0.9 .... 2.74 0.72 0.44 0.16 An19 1.05
26.29 28.01 36.22 5 0.77 .... 2.11 0.58 0.38 0.23 An12 1.04
the southern part of the eastern Karakoram. The batholith represents a morphologically elevated region to the north of the Nubra-Shyok valleys, extending almost parallel to the
Shyok suture (figures 1 and 3). In several places, the sharp contact between the Shyok suture and the Karakoram batholith is defined by the NW–SE trending Karakoram
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strike-slip fault zone, which is punctuated by hot springs (figures 1 and 5). The southern boundary of the Karakoram batholith is defined by mylonites and a ~ 50-m-wide zone of strongly foliated carbonaceous slates, marbles, metaconglomerates, quartzites, micaschist and gneisses. Near the Ophiolitic melange the Karakoram granitoids are leucocratic, coarse-grained, porphyritic orthogneisses. The gneissic character of these rocks decreases northward and the rocks gradually pass into porphyritic granites and granodiorites. The most common rocks of the Karakoram batholith are weakly to moderately deformed muscoviteand biotite-bearing two-mica granites, hornblende-biotite granitoids and medium- to coarse-grained K-feldspar rich granites, enclosing large xenoliths of metasedimentary and mafic rocks. Compositionally, the Karakoram batholith includes granite to quartz monzonite, granodiorite, and tonalite (figure 12). Aplites, pegmatite dykes, fine-grained quartz-feldspathic veins and dykes of intermediate composition are common. The granitic batholith intruded the Carboniferous–Permian sequence of the Karakoram Tethyan zone to the north [31]. 2.6.1. Geochemistry of the Karakoram batholith Eighteen representative samples of Karakoram granitoids were analysed for major, trace and rare earth elements (REE); nine representative analyses are given in table II. The analytical procedures are the same as for the Tirit granitoids. The Karakoram granitoids have a range of SiO2-content from 61.2 to 72.95 wt% (table II). They further show a wide range in Al2O3 (14.77 to 16.67 wt%) and CaO (1.14 to 4.32 wt%). The major oxide versus SiO2 plots show a systematic decrease in TiO2, Al2O3, Fe2O3(t), MnO, MgO, CaO and P2O5 with increasing values of silica (figure 15). According to the AFM and QBF diagrams (figures 13, 14) the Karakoram granitoids are following both calcalkaline and subalkaline trends. The higher A/CNK values than the Tirit granitoids ranging between 0.96 to 1.23 further suggest both a metaluminous and peraluminous nature (table II, figure 17). According to the SiO2 versus A/CNK diagram (figure 17) the Karakoram granitoids are both I-and S-types. Trace elements versus SiO2 variation diagrams (figure 16) show that Rb, Y, Zr, Sr and Nb systematically decrease with increasing values of silica; Pb shows a good negative correlation. Chondrite-normalised REE patterns (figure 21a, b) suggest that all samples are strongly enriched in Light Rare Earth Elements (LREE) (La = 31.1–113.1 × Chondrite) and depleted in Heavy Rare Earth Elements (HREE) (Yb = 0.56–1.76 × Chondrite). This indicates that the Karakoram granitoids are more LREE enriched than the Tirit granitoids. All the samples are highly fractionated in their total REE contents with a (La/Lu)N ratio of 12.95 to 56.94. Similarly, samples K4 and K20 (table II) from the Karakoram granitoids show a significant enrichment in Rb, Ba, Large Ion Lithophile Elements (LILE) and a strong deple-
tion in High Field Strength Elements (HFSE), which suggests a pattern similar to that of syn-collision granitoids. The primitive mantle normalised trace-element patterns (figure 22) show a systematic depletion in Ti, P, Sr, Nb and Ba. This depletion is typical of a calcalkaline magmatism in a subduction zone environment. The Nb versus Y, and Rb versus Y+Nb diagrams (figure 20) further confirm that the Karakoram granitoids are both volcanic arc granites (VAG) and syn-collision granites (Syn-COLG), hence comparing to the continental arc plutonism of the Chile arc [32]. We have dated three samples of the S-type granite collected from the middle part of the eastern Karakoram batholith by using Rb/Sr isotopic whole rock technique. This S-type granite is 83 ± 9 Ma old with an initial 87Sr/86Sr ratio of 0.7994 ± 0.00023 [33, 34]. This age suggests that collision between Kohistan-Ladakh arc and Karakoram block was active 83 ± 9 Ma ago.
3. Conclusion Similarity exists between the Shyok suture of northern Ladakh and the Northern suture of Kohistan. It is likely that the Kohistan and the Ladakh units evolved as a single tectonic domain during the Cretaceous–Palaeogene. We also support Petterson and Windley’s [11] interpretation and suggest that the Shyok suture is older than the Indus suture and closed sometimes between 100–75 Ma. The accretionary processes in the Karakoram region began prior to the final closure of the Indus suture. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture zone (50–60 Ma) and the formation of the Nanga ParbatHaramosh syntaxis separated Kohistan and Ladakh. The Holocene–Recent dextral offset along the Karakoram fault [2] reshaped and rejuvenated the tectonic structures and the architecture of the entire Karakoram, the Shyok suture and the adjoining Indian plate region. Acknowledgements. We are grateful to the Department of Science and Technology, Govt. of India, New Delhi for providing financial assistance under the sponsored Project No. ESS/CA/A9-32/93. We thank the authorities of the Wadia Institute of Himalayan Geology, Dehradun for extending their support. Sincere thanks to Profs Brian Windley, Maurizio Gaetani, Daniel Bernoulli, J.P. Burg and Drs M.G. Petterson, Wilfried Winkler and two anonymous reviewers for their critical reviews and encouragement on an earlier draft of this manuscript which formed the base of the present paper. Thanks are due to Drs P.P. Khanna, N.K. Saini of XRF and ICP-Labs of WIHG for analysing the granite samples and generation of other geochemical data. Rb/Sr dates originated from PRL Ahmedabad, India, under the guidance of Dr J.R. Trivedi. RU thanks the Federal Commission of Scholarships, Switzerland, for providing a fellowship to carry out research at the Institute of Geology,
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a b Figure 21 a and b. Chondrite-normalised REE plots for the Karakoram granitoids (normalised values after [27]).
ETH Zurich, Switzerland, and to Profs Daniel Bernoulli and J.P. Burg for providing every kind of facility, support and encouragement at ETH Zurich.
Figure 22. Primitive mantle normalised trace element plots for the Karakoram granitoids (normalised values after [27]).
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